selftests/bpf: Convert test printk to use rodata.

Convert test trace_printk to more aggressively validate and use rodata.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-21-alexei.starovoitov@gmail.com